1. Field of the Invention
The invention relates to an SiO2 shaped body which is vitrified in partial areas, to a process for producing it, to its use, and also to a device suitable for manufacture of such SiO2 shaped bodies.
2. Background Art
Porous, amorphous SiO2 shaped bodies are used in numerous technical fields. Examples which may be mentioned include filter materials, thermal insulation materials or heat shields.
Furthermore, all kinds of quartz products can be produced from amorphous, porous SiO2 shaped bodies by means of sintering and/or fusion. High-purity porous SiO2 shaped bodies can in this context be used, for example, as preforms for glass fibers or optical fibers. Furthermore, in this way it is also possible to produce crucibles for pulling single crystals, in particular silicon single crystals.
In prior art methods for sintering and/or fusing quartz products, for example, furnace sintering, zone sintering, arc sintering, contact sintering, sintering using hot gases or by means of plasma, the quartz products which are to be sintered and/or fused are heated by the transfer of thermal energy or thermal radiation. If the quartz products produced in this way are to have extremely high purity with regard to any type of foreign atoms, the use of hot gases or hot contact surfaces leads to undesirable contamination to the sintered and/or fused quartz product with such foreign atoms.
Therefore, in principle, contamination with foreign atoms can only be reduced or avoided by non-thermal, contactless heating by means of radiation. Contactless heating by means of radiation under standard pressure is possible, for example by sintering or fusion of an open-pored SiO2 green body with the aid of a CO2 laser beam. However, a significant drawback of this method is the quality of the vitrified regions. If an open-pored porous green body is sintered or fused using a laser beam, a large number of gas inclusions, or as “gas bubbles,” are formed. These cannot escape or can only escape with difficulty, on account of the high viscosity of the molten amorphous glass phase. As a result, therefore, the vitrified layer contains a large number of such gas inclusions.
If high-purity quartz glass products such as crucibles used to pull single crystals, in particular silicon single crystals, are produced in this way, the gas inclusions on the inner side of the pulling crucible cause considerable problems during the crystal pulling process with regard to the yield and quality of the silicon single crystal. Furthermore, during the crystal pulling process, gas bubbles originally produced under standard pressure grow to a considerable extent under reduced pressure used during the pulling process. This leads to considerable problems caused by contamination with what is known as CVD cristobalite if the large gas bubbles open up during the pulling process.